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Several people have been confirmed injured in a swarm of earthquakes that have struck Christchurch this afternoon, with six shakes since 1pm causing damage all around the city.
The largest earthquake, a 6.0, struck at about 2.20pm.
The first shake hit at 1pm and was a magnitude 5.5 quake, but has been eclipsed by the larger event.
It was felt as far away as Wellington and Invercargill and has seen Christchurch Airport evacuated, with reports that one runway may have been damaged in the shakes.
The Christchurch Airport terminal has reopened, but runway checks are continuing. Passengers should expect delays and contact their airlines for flight information.
Fire Service spokesman Ian Lynn said a bridge has reportedly come down following the magnitude 6.0 quake - believed to possibly be on Gayhurst Road, Avonside.
He said there have been further callouts, mostly for broken water mains.
"This place is going mad."
Police are evacuating the central city red zone after reports of a possible gas leak; and the Old Waimakariri bridge, Moorhouse Ave overbridge, Gloucester/Gayhurst bridge and Anzac Ave bridge are all closed.
The Merivale Mall has also been closed and the central police station has been evacuated.
The historic Lyttelton Timeball Station - which suffered significant damage in the February 22 earthquake, has collapsed.
Since the 6.0 shake, there has been two further earthquakes, with a 4.9 magnitude quake at 2.40pm and a 3.7 quake at 3.08pm.
The quakes have all been in the same area, about 10km east of the city and 10km deep.
Injuries and rescues
Two people have been rescued from a collapsed church in the centre of Christchurch and several people have been taken to hospital in the wake of the city's latest shake, but there have been no reports of any serious injuries.
Sarah Martin of St Johns ambulance said six people have been transferred to hospital with "moderately serious" injuries.
Mr Lynn said two people have been rescued from the St John Church on Hereford and Madras streets, but they are not believed to be seriously injured.
The city was first struck at 1pm by a powerful 5.5 magnitude earthquake which was felt as far south as Dunedin.
The quake was at a depth of 11km, centred 10km south of Christchurch, off the coast of Sumner.
It was followed by a magnitude 4.4 aftershock eight minutes later, which struck in the same area and depth as the first shake, and another 3.4 quake at 1.28pm.
Police spokesman Stephen Hill said there had been no reports of serious injuries sustained in the aftershocks.
He said officers had cleared a collapsed unit on the corner of Stanmore and Worcester St and found no casualties.
Police were attending emergencies "right across the city", with most calls focused on liquefaction damage in suburbs and building damage in the city centre, he said.
Calls to the Fire Service are coming largely from the coastal areas. All resources in the area are responding, including volunteer brigades. The Christchurch Urban Search and Rescue team has been deployed.
Herald reporter Jarrod Booker, who was at the Riccarton Racecourse attending the coroner's inquest into 106 deaths at the CTV building during the February 22 quake, said there was severe shaking with the ground "moving side-to side" during this afternoon's first shake.
"People started running out of the building, they were quite frightened."
The first shake was felt in Ashburton, Timaru and Dunedin.
There have been reports of liquefaction bubbling up from the ground in the central Christchurch suburb of Richmond and the outer suburbs.
Rocks have been seen falling from the hills in Sumner, while the University of Canterbury and Riccarton Mall have both been evacuated.
Lyttelton Tunnel closed briefly, but has reopened.
The Christchurch City Council say residents in Heberden Avenue, Sumner have been asked to self-evacuate for their own safety.
The council says it is still assessing the situation followibng today's large aftershocks.
People are advice to limit their travel to ease congestion to make it easier for emergency service to travel.
A caller told Newstalk ZB's Danny Watson the Centra Hotel opposite the Grand Chancellor is on a lean.
"It appears as though one of the walls on the western side is probably on a lean of 5 to 10 degrees.
"It looks out of place by 5 - 8 metres from the top down to the bottom."
Another caller said that water is pouring out of buildings on Stanmore Road.
"It's a funny colour. It looks like sewage."
One Facebook user said they were "not enjoying being up the hill above sumner right now... Saw half a bloody cliff fall from that shake".
Power and phone
Power and phone services have been cut around Christchurch.
The earthquake has impacted both the cellphone and landline network. Telecom says it is assessing any potential network impact.
Telecom is also urging customers to text word messages, rather than pictures or video, and to limit voice calls to help conserve load on the network.
Telecom said there was some evidence of overloading on its network "as you would expect when there has been a decent shake".
Orion NZ says power has been cut to around 54,000 customers across Christchurch following the magnitude 6 earthquake.
Orion reports Spreydon, Sumner, Redcliffs, Heathcote, New Brighton, Dallington, Fendalton and St Albans have all been affected by power cuts.
Eleven Countdown supermarkets across Christchurch have been closed following this afternoon's earthquake
"Our priority at this early stage is the safety of our staff and our customers. We are currently assessing damage to stock and cleaning up as necessary," the company said.
Countdown Colombo St is closed until further notice, while Countdown's Eastgate store has been evacuated with re-entry subject to engineer checks.
Countdown's Kaiapoi, Avonhead, Moorhouse, New Brighton, Northwood and Church Corner stores are expected to reopen tomorrow. Other stores in the group, including Bush Inn, Northlands and Rangiora are expected to open later today.
Countdown Ferrymead and The Palms remain closed from the February 22 earthquake. Countdown Hornby is still trading.
The Ministry of Civil Defence and Emergency Management has activated the National Crisis Management Centre following today's major aftershocks.
National controller, David Coetzee, said the response to the aftershocks is being managed from the Christchurch City Council's Emergency Operations Centre.
The police headquarters and the offices of the Christchurch Earthquake Recovery Authority have been evacuated.
Christchurch East MP Aaron Gilmore said he drove from his office to his home in his electorate after the 5.5 magnitude quake.
He said the areas which were heavily hit by liquefaction after the February 22 earthquake - Parklands, New Brighton, Bexley, Aranui - once again have flooded streets.
"There is mud, liquefaction water and potholes all along the streets that have already been repaired in the last few months.
"The poor guys who have been doing all that work - well they're out there already."
"It is pretty serious.
He said the magnitude 6 quake felt much stronger.
"There will certainly be some injuries from these quakes - hopefully there are no major serious injuries."
More damage at Cathedral
Christchurch Cathedral dean Peter Beck said the "very big" aftershock had shaken his temporary offices above Christ's College.
Students had been evacuated and were gathering on the school fields, he said.
He had received reports about damage to homes and more damage to the city centre, but no word of injuries.
"We just hope and pray that no-one's been hurt.
"It's a hard time but at the same time we are an extremely resilient people. We will live through it. We will continue to recover and rebuild our city."
The aftershock would have inflicted further damage on the severely earthquake hit Christchurch Cathedral, Mr Beck said.
Stronger 'peak ground acceleration'
GNS Science seismologist Jennifer Coppola said the first earthquake would have felt stronger than last Monday's 5.5 magnitude aftershock, as it had a stronger peak ground acceleration.
Ms Coppola said GNS Science had forecast between one and 10 aftershocks between magnitude 4 and 4.9, and between zero and two larger than magnitude 5 for Canterbury between May 19 and June 18.
"So we've hit the magnitude 5 and above number - hopefully there are no more."
Residents have taken to Twitter to post about the quake.
Lilmagoolie said her house "wobbled like crazy" - with the shaking almost as bad as the February shock.
"Big quake. House still moving. Alarms going off everywhere."
Mattdeevee said: "oh my gosh that was so scary!!!!!! Biggest since Feb by far 5.9?!"
Nathanael Boehm, NathanaelB, said he was ready to jump out a
"Was up and ready to leap out a window if it started coming down!"
Joy Reid of TVNZ tweeted about being in Riccarton mall: "Stuff fell off shelves.people crying.everyone leaving," she tweeted.
Earlier today a 4.3-magnitude quake hit near Darfield in central Canterbury.
View Big aftershock hits Christchurch in a larger map
- NZ HERALD STAFF